jenkins debugging of SCM
Your jobs will have an SCM polling log, e.g. http://jenkinshost/job/jobname//scmPollLog, which will tell you if the execution of the SCM polling is having problems such as not being able to find the SCM client executable.
The Jenkins master log may also show errors - the location of this log depends on how you've installed Jenkins, so if you can't find it add a comment here and I'll try to help you out.
You should also double check the cron expression you're using in the SCM polling section of your Jenkins job, just to make sure it's really firing on the schedule you want. You can test cron expressions with tools such as http://www.cronmaker.com/
There is an issue in Jenkins about polling issues.
It might be caused because Jenkins is in "shutdown mode". In this case the scm polling is skipped.